This invention relates to a process for a direct reduction of iron oxide containing materials to form sponge iron in a rotary kiln, wherein the charge is moved through the rotary kiln countercurrently to a gas atmosphere, solid carbonaceous reducing agent having a high content of volatile constituents is charged into the rotary kiln at its charging end, solid carbonaceous reducing agent having a high content of volatile constituents is blown into the rotary kiln at its discharge end and is distributed over the charge in part of the length of the kiln, and oxygen-containing gases are introduced into the rotary kiln through its shell at a plurality of locations.
That zone of the rotary kiln which directly precedes its discharge end is particularly susceptible to temperature fluctuations and to a depletion of carbon in the charge bed. This may result in fluctuations of the degree of metallization, in a formation of deposits in the kiln and in a higher heat consumption.
Various proposals have been disclosed as to how part of the solid carbonaceous reducing agent can be pneumatically blown into the rotary kiln at its discharge end and can be distributed over the charge in part of the length of the kiln.
From U.S. Pat. No. 981,280 it is known to blow pulverulent bituminous coal by means of air through a first blowing device, which acts like a burner for pulverized coal. A second blowing device is used to blow lump coal having a particle size of 10 to 20 mm through a second blowing device.
U.S. Pat. No. 4,378,244 teaches blowing 10 to 35% of the entire coal as a fraction having a particle size from 0 to 12.5 mm into the kiln at its discharge end by two blowing devices and distributing said coal over at least 50% of the length of the kiln. The remaining coal having a particle size up to about 50 mm is charged into the kiln at its charging end.
Canadian Patent No. 872,728 teaches blowing coal having a particle size up to 6 mm into the kiln at its discharge end by means of a blowing device and charging the remaining coal into the kiln at its charging end, with the exception of coal which has a particle size below 1 mm and which is also blown into the kiln at its discharge end.
Indian Patent Specification No. 142,368 teaches separating the entire coal at a sieve cut of 6 mm, charging the coarse fraction into the kiln at its charging end and blowing the fine fraction into the kiln at its discharge end. The fine fraction is blown in by one or two blowing devices at a rate of 40 to 70% of the total rate.
Published German Application No. 33 32 556 teaches blowing 60 to 100% of the fresh feed coal into the kiln at its discharge end and states that the blown coal must not contain more than 20% of that fine fraction, which has a particle size below 4 mm and is contained in the entire amount of fresh feed coal. The coal is blown into the kiln by two blowing devices. One blowing device is used to blow part of the coal which has passed through the sieve and contains the permissible amount of the fraction having a particle size below 4 mm and to distribute said part of the coal over not more than 20% of the length of the kiln. A second blowing device is used to blown coal which has been retained on the sieve and has a particle size above 4 mm. That coal is distributed over the length portion of the kiln between points which are spaced 20 and 70% of the length of the kiln from the discharge end.
These above mentioned processes, however, cannot always maintain optimum operating conditions in the final zone of the rotary kiln.